Facile, low-energy routes for the production of hydrated calcium and magnesium salts from alkaline industrial wastes

ABSTRACT

Divalent ions are extracted from solids by leaching to form a divalent ion-containing solution. The divalent ion-containing solution is subjected to concentration to form a concentrated divalent ion-containing solution. Precipitation of a divalent ion hydroxide salt is induced from the concentrated divalent ion-containing solution. In other cases, the concentrated divalent ion-containing solution is exposed to carbon dioxide to induce precipitation of a divalent ion carbonate salt.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Entry of International ApplicationNo. PCT/US2018/046981, filed Aug. 17, 2018, which claims the benefit ofand priority to U.S. Provisional Application No. 62/547,451, filed Aug.18, 2017, and the benefit of U.S. Provisional Application No.62/679,498, filed Jun. 1, 2018, the contents of which are incorporatedherein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under DE-FE0029825,awarded by the U.S. Department of Energy, and 1253269, awarded by theNational Science Foundation. The Government has certain rights in theinvention.

TECHNICAL FIELD

This disclosure generally relates to the production of portlandite andbrucite and the production of carbonate solids.

BACKGROUND

The commercial production of portlandite (Ca(OH)₂) and brucite (Mg(OH)₂)presently involves calcination-based methods, which require asignificant amount of thermal energy. To produce portlandite, calcite(CaCO₃) is heated to about 800° C. to produce lime (CaO) and carbondioxide (CO₂). The lime produced is then reacted with water to formCa(OH)₂ by a slaking process. Commercial Mg(OH)₂ is produced by addingportlandite to seawater rich in Mg²⁺, thereby precipitating Mg(OH)₂.Alternatively, hydrated magnesium chloride (MgCl₂) from seawater isrecrystallized several times, and then calcined at about 1000° C.,before reacting with water to form a hydroxide slurry.

Also, brines from various industrial processes, such as desalinationoperations and oil and gas production, are hypersaline streams typicallytreated as waste. However, such brines are actually a potential sink forCO₂ as they are rich in divalent ions such as Ca²⁺ and Mg²⁺ which reactwith aqueous CO₃ ²⁻ from the dissolution of CO₂ in water to formrelatively insoluble carbonate solids. Carbonate precipitates formed inthis manner can have a wide range of industrial applications. An exampleof a brine waste stream is produced water, which refers to the watergenerated during oil and gas production, and which can have divalent ionconcentrations of up to about 100,000 parts per million (ppm). Suchprocess can provide an effective route to mitigate the increasinganthropogenic CO₂ emissions at a gigaton-scale, since brines aregenerated at large quantities annually around the world (e.g., about 22billion barrels of produced water was generated in 2014 in the UnitedStates alone). However, effective extraction of divalent ions fromhypersaline and complex aqueous matrices in brines as carbonate solidsat low cost and energy expense remains a significant challenge.

It is against this background that a need arose to develop theembodiments described herein.

SUMMARY

In some embodiments, a method includes: (1) extracting divalent ionsfrom solids by leaching to form a divalent ion-containing solution; (2)subjecting the divalent ion-containing solution to concentration to forma concentrated divalent ion-containing solution; and (3) inducingprecipitation of a divalent ion hydroxide salt from the concentrateddivalent ion-containing solution.

In some embodiments, a method includes: (1) producing a concentrateddivalent ion-containing solution from a feed solution; and (2)introducing carbon dioxide to the concentrated divalent ion-containingsolution to induce precipitation of a carbonate salt.

In some embodiments, a system includes: (1) a divalent ion enrichmentunit to produce a concentrated divalent ion-containing solution from afeed solution; and (2) a carbonation reactor connected to the divalention enrichment unit to receive the concentrated divalent ion-containingsolution and introduce carbon dioxide to the concentrated divalention-containing solution to induce precipitation of a carbonate salt.

Other aspects and embodiments of this disclosure are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict this disclosure to any particular embodiment but aremerely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof this disclosure, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 . A schematic of a capacitive concentration process.

FIG. 2 . (a) Effect of flow rate and cycle time on ion concentration forthe following operation parameters: about 1 V and 0 V for charging anddischarging respectively, inlet concentration of about 10 mM CaCl₂ andcarbon electrodes. Flow rates and cycle times are specified in thelegend. (b) Evolution of normalized concentration over time measured for3 potential cycling sequences (about 1 V and 0 V) for an about 10 mMCaCl₂ inlet solution, a flow rate of about 0.1 mL/min and a cycle timeof about 6 h.

FIG. 3 . Examples of 1- and 2-pass processes of capacitive concentrationresulting in a Ca(OH)₂-saturated solution, optimized based on Caconcentration in an inlet solution.

FIG. 4 . A schematic of a membrane filtration process.

FIG. 5 . Results of reverse osmosis experiments with about 5 mM andabout 10 mM Ca(OH)₂ solutions showing (a) normalized calciumconcentration as a function of time for concentrate and permeate streamsand (b) Ca rejection as a function of water recovery.

FIG. 6 . Thermodynamic calculations on portlandite precipitation. Inthis simulation carried out in PHREEQC using the minteq.v4 database, thetemperature of a solution that is initially at saturation with respectto Ca(OH)₂ at 25° C. was increased incrementally to 90° C. Because ofthe retrograde solubility of portlandite, namely solubility decreaseswith increasing temperature, supersaturation of the initial solution isachieved. (a) Moles of Ca(OH)₂ precipitated per kg of an initiallyCa(OH)₂-saturated solution at 25° C. As the temperature is increased,the solution becomes increasingly supersaturated in portlandite,resulting in an increasing extent of precipitation of the solid. (b)[Ca] and pH of the same solution as in (a) decrease with increasingtemperature as portlandite precipitates.

FIG. 7 . Process schematic of brine solution enrichment and subsequentcarbonation of enriched solutions using (a) membrane filtration and (b)capacitive concentration.

FIG. 8 . Carbonation of calcium-containing brine solution andcorresponding pH change. Conditions are as follows: Initial solutionvolume=about 1.0 L, [CaCl₂]_(initial)=about 12.5 mM, [NaCl]=about 1.0 M,CO₂ injection flow rate=about 100 mL min⁻¹, pH=about 12 adjusted byNaOH, temperature=about 20° C., and pressure=about 1.0 atm.

DETAILED DESCRIPTION Production of Hydrated Calcium and Magnesium Saltsfrom Alkaline Industrial Wastes

Some embodiments of this disclosure are directed to a thermally and CO₂efficient route for the production of hydrated divalent ion salts,including the production of hydroxide salts of calcium and magnesium,namely portlandite (Ca(OH)₂) and brucite (Mg(OH)₂). In some embodiments,the method can be performed at ambient to moderately elevatedtemperatures at or below about 100° C. In some embodiments, the methodutilizes industrial wastes or by-products, such as those derived frommetal processing and fuel combustion, and other sources that aregenerally enriched in calcium (Ca) and magnesium (Mg), and that providea route for the production of hydrated calcium and magnesium saltswithout calcination.

In some embodiments, a method for Ca(OH)₂ and Mg(OH)₂ productioninvolves the following operations: (1) extraction of calcium andmagnesium divalent ions (cations in the form of Ca²⁺ and Mg²⁺) from asource of Ca and Mg, such as crystalline or amorphous alkaline solids(e.g., included in industrial wastes or by-products, among others) byleaching to form a Ca- and Mg-containing leachate; (2) concentration ofthe Ca- and Mg-containing leachate by capacitive concentration (CC) ormembrane filtration to form a Ca(OH)₂- and Mg(OH)₂-saturated solution,and (3) direct precipitation of Ca(OH)₂ and Mg(OH)₂ from the solution byexploiting the temperature-dependent solubility of these salts.

In operation (1) according to some embodiments, Ca²⁺ and Mg²⁺ areextracted from industrial wastes or by-products, such as those derivedfrom metal processing and fuel combustion (e.g., coal fly ashes), amongother sources that are generally enriched in Ca and Mg. In someembodiments, Ca²⁺ and Mg²⁺ are extracted from slags, fly ashes, or otheralkaline solids by dissolution in, or exposure to, water or otheraqueous leaching solution at ambient or moderately elevated temperature,and at ambient pressure, in the presence or not, of specific leachingaids. Slags, which are by-products derived from metal production,include slags derived from iron production (e.g., air-cooled blastfurnace (BF) slag) and steel production (e.g., electric arc furnace(EAF) slag and basic oxygen furnace (BOF) slag), and are typicallycomposed of Ca- and Mg-containing silicates. Although glassy slags finduse as replacement material for ordinary Portland cement (OPC),crystalline slags presently find limited use as low-value aggregates.Such crystalline slags are abundant and include significant amounts ofCa and Mg. Fly ash, including that sourced from historical reservoirs(e.g., landfills and ash ponds), is a coal combustion by-product whichalso includes high concentrations of Ca. In order to increase a leachingrate, one or more metal leaching agents (e.g., acetate,ethylenediaminetetraacetic acid (EDTA), and so forth) and/or one or moreacids (e.g., acetic acid, hydrochloric acid, and so forth) can be addedto a leaching solution. A slag can also be ground or pulverized to finerparticle sizes to increase the rate of light metal extraction.

Next, in operation (2) according to some embodiments, concentrations ofCa and Mg in a leachate are enhanced using CC, leveraging an increasedselectivity of CC to divalent ions. In some embodiments, a CC cellincludes a capacitor arrangement having the following features: a smallthickness, a high specific surface area, a high porosity, and a bimodalpore size distribution between mesopores and micropores in itsstructure. In some embodiments, a material of an electrode in thecapacitor arrangement is an inert carbon-based or carbonaceous solid,which acts as a current collector, and is resistant to corrosion. A CCcell operated at a relatively low electrical input can be more energyefficient than certain membrane processes driven by high-pressure pumps.

A schematic of a CC process is shown in FIG. 1 . As shown, an inletstream is conveyed through a CC cell (as a divalent ion enrichment unit)using a pump (e.g., a peristaltic pump), and the CC cell includes a pairof electrodes. Concentration enhancement of metals in a solution iscarried out by applying an electrical input on the pair of electrodes inthe form of voltage or electric current, where the electrical input isapplied via an electrical source connected to the electrodes.Consequently, metal ions present in the solution migrate to surfaces ofthe electrodes, forming an electric double layer (EDL) structure.Generally, an electrically charged surface that is in contact with anelectrolyte solution attracts ions in the solution of an oppositecharge. Hence, positive and negative ions migrate and are adsorbed atthe negatively and positively charged electrode, respectively, producinga relatively ion-depleted stream of the solution. When the concentrationof ions in an outlet stream reaches a threshold/pre-determined value,the current or voltage is reversed or is reduced to zero, releasing theions adsorbed at the surfaces of the electrodes back into the solution.Thus, a solution enriched with ions (relative to an original inletsolution) is obtained. For example, for calcium chloride solutions, anion concentration in an outlet solution is about 20% to about 80% higherthan in an inlet solution in terms of normalized concentration (e.g., aratio between the outlet and inlet concentrations) depending on flowrate and cycle time, as shown in FIG. 2 a . In general, longer cycletimes and lower flow rates can enhance the concentration factor.Further, cyclic CC demonstrates the repeatability of the concentrationprocess (e.g., adsorption-desorption of ions), as shown in FIG. 2 b.

Referring back to FIG. 1 , the pair of electrodes of the CC cell areconnected to a potentiostat, which is used to apply voltage cycles andobtain a measurement of a current response in the cell, while solutionconductivity is measured for an outlet stream using a conductivity meterto determine the extent of concentration. A data acquisition (DAQ)system is connected to various components to direct operation of the CCprocess.

Depending on an initial leachate concentration, a series of connected CCcells can be included in a CC system to achieve a desired concentrationfor Ca(OH)₂ and Mg(OH)₂ saturation at about 25° C., which are calculatedto be about 21 mM and about 0.13 mM in pure water, respectively.Therefore, according to these assumptions, the number of passes throughone or more CC cells can be optimized as a function of the concentrationof Ca (or Mg) in an inlet solution (FIG. 3 ). In general, the number ofpasses can be n, where n is an integer that is 1 or more, such as 2 ormore, 3 or more, and so forth. It is also contemplated that multiplepasses can be performed by recirculating an outlet stream from a CC cellas an inlet stream for the same CC cell, for concentration enhancement.

Unlike capacitive deionization (CDI) systems which are typically used todecrease ion concentration in solutions in desalination systems,performing the CC operation in the method of some embodiments isspecifically designed to increase a concentration of ions in an outletstream, thereby converting a dilute stream of Ca and Mg ions into onethat is more concentrated, such as one that is at or near saturationwith respect to Ca(OH)₂ and Mg(OH)₂. Particularly, since the CC systemis operated at or near ambient temperature and pressure, performing theCC operation features low energy consumption, in contrast to otherapproaches such as concentration by solvent evaporation, which involveshigh temperature.

Further, the divalent Ca and Mg ions can be selectively concentratedusing CC, by leveraging a transient ion selectivity that arises when anEDL at a surface of an electrode reaches equilibrium with an appliedelectrical input. As the input is applied, ions that are most abundantin a solution will initially migrate to an electrode. However, at latertimes, the adsorbed ions will be replaced by those of a higher valence.

Another option for enhancing concentrations of Ca and Mg in a leachateis using membrane filtration. Membrane filtration is a separationprocess carried out by pressurizing a solution including ions and otherimpurities through a semipermeable membrane, such as a nanofiltrationmembrane or a reverse osmosis membrane, which can be functionalized. Aless concentrated stream (namely, a permeate) passes through thesemipermeable membrane, while dissolved salts and other impurities areretained in a concentrated stream (namely, a retentate). The rejectionof ions is due to one or more of size exclusion, charge exclusion, andphysical-chemical ion interaction effects between the solution and thesemipermeable membrane. The pressure to drive the membrane filtrationprocess depends on the concentration of ions in an inlet stream.

A schematic of a membrane filtration process is shown in FIG. 4 . Asshown, an inlet stream is conveyed into a feed tank, and a feed solutionis then conveyed from the feed tank through a membrane separation unit(as a divalent ion enrichment unit) by operation of a pump and a valvein a cross-flow mode: a retentate is conveyed back to the feed tankwhile a permeate passes through a membrane in the membrane separationunit. The feed solution is continuously concentrated as the divalentions (e.g., Ca²⁺ and Mg²⁺) are retained and accumulate in an outletstream. Using calcium hydroxide solutions, Ca concentration factors (interms of a ratio between the outlet and inlet concentrations) greaterthan about 2 can be obtained using reverse osmosis membrane filtration,corresponding to Ca rejection greater than about 90% (FIG. 5 ).

Next, in operation (3) according to some embodiments, subsequentprecipitation of Ca(OH)₂ and Mg(OH)₂ involves a temperature swingprocess. An outlet solution from the CC operation can be conveyed to areactor, and can be heated (e.g., to a temperature above ambienttemperature or above about 25° C. and at or below about 100° C., such asup to about 95° C., up to about 90° C., up to about 80° C., up to about70° C., or up to about 60° C.) using, for example, waste heat from apower plant or other thermal source. Because the solubilities of Ca(OH)₂and Mg(OH)₂ decrease with increasing temperature, such a temperatureincrease results in the supersaturation of the solution with respect tothese salts, and eventual precipitation of these salts from thesolution. It is noted that if leaching is performed at an elevatedtemperature, a leachant is first cooled to at or near about 25° C. priorto the CC operation to allow for higher aqueous Ca and Mgconcentrations. Equilibrium calculations show that up to about 7 mmol(about 0.52 g) of portlandite can be precipitated with the method fromabout 1 kg of water that was at saturation with Ca(OH)₂ at about 25° C.(FIG. 6 ).

Selectively Enriching Divalent Ions from Brines for CO₂ Mineralization

Effective extraction of divalent ions from hypersaline and complexaqueous matrices in brines as carbonate solids at low cost and energyexpense remains a significant challenge. This process of CO₂mineralization can be significantly improved by utilizing precursorscontaining divalent ions at relatively high concentrations. Methods thatcan selectively enrich divalent ions such as Ca²⁺ and Mg²⁺ forsubsequent CO₂ mineralization are therefore particularly desired tofacilitate carbonate formation while deriving valuable products.

Some embodiments of this disclosure are directed to enhancing theefficiency of CO₂ mineralization processes that utilize industrial andnatural brines, including produced water from hydraulic fracking, brineconcentrate from desalination plants, industrial wastewater, and seawater. This is achieved by implementing a brine pre-treatment stage toselectively enrich divalent ions (e.g., Ca²⁺ and Mg²⁺), therebysubstantially increasing the carbonation capacity of brine waste streamswhile reducing operational costs.

During CO₂ mineralization using brines, carbonate solids precipitatefrom a solution in the form of polymorphs of calcium carbonate,magnesium carbonate, and related compounds. Precipitation reactions canbe described as the reduction of soluble ions from the solution throughthe formation of an insoluble product. Yet, precipitation of thesecarbonates from brines may not be favored because of the high backgroundsalinity, arising from the abundance of alkali metals and halides (e.g.,Na⁺, K⁺, and so forth), which typically occurs at a much higherconcentration than that of Ca²⁺ and Mg²⁺. Comparable pathways for directcarbonation using brines involve substantial consumption of energy(e.g., electrolysis or temperature swing) or use of expensive chemicalreagents (e.g., HCl or NaOH for pH adjustment), rendering such processeseconomically unfeasible.

Towards this end, some embodiments incorporate a pre-treatment stage ina CO₂ mineralization process to selectively enrich divalent ions inbrines. To reduce operational costs, economically practicable enrichmentoptions, namely membrane filtration (e.g., nanofiltration or reverseosmosis), and/or capacitive concentration (CC), may be selected. Theseconcentration processes can accommodate natural or industrial brineshaving various pHs and containing various aqueous species of a widerange of concentrations. In addition, each of these processes can beoptimized for operation under a wide range of processing conditionsincluding flow rate, cycle time, and so forth.

Membrane filtration is a rapid enrichment process characterized byexcellent selectivity towards divalent ions such as Ca²⁺ and Mg²⁺ (orother multivalent ions) over monovalent ions such as Na⁺ and K⁺,afforded by proper selection of the membrane pore size. For example, amembrane having a pore size of about 0.5 nm to about 10 nm can retaindivalent ions (e.g., Ca²⁺ and Mg²⁺) in a retentate while allowing thepassage of smaller monovalent ions (e.g., Na⁺ and K⁺) in a permeate whena filtration (e.g., nanofiltration) unit is operated in a cross-flowmode. Therefore, the divalent ions accumulate in the retentateultimately reaching high concentrations. The retention percentage can beup to about 99% (or more) for Ca²⁺ and Mg²⁺, yet there can be also ahigh rejection rate of greater than about 50% for Na⁺ and K⁺. In someimplementations, the membrane can have a pore size that allows about 40%to about 50% permeation for divalent ions (e.g., Ca²⁺ and Mg²⁺), butclose to 0% retention for monovalent ions (e.g., Na⁺ and K⁺), such asabout 10% retention or less, about 5% retention or less, or about 1%retention or less. For example, the pore size can be about 1 nm orgreater, about 3 nm or greater, or about 5 nm or greater, and up toabout 10 nm or greater. The reduced NaCl level in the retentate canfurther facilitate CO₂ mineralization since high NaCl concentrations caninhibit carbonate precipitation.

CC is another option for selective enrichment of divalent ions.Electrodes of a CC cell can exhibit higher selectivity to divalent ions(e.g., Ca²⁺ and Mg²⁺) over monovalent ions (e.g., Na⁺ and K⁺), becauseof their higher charges. Under optimized conditions (e.g., in terms ofelectrode configuration, electrical input, and flow rate), up to about90% (or more) of divalent ions can be separated from a solution.

Following pre-treatment of a brine by either membrane filtration or CC,carbonation of a concentrated solution including divalent ions canproceed with a significantly enhanced efficiency.

FIG. 7 shows a schematic of a pre-treatment and carbonation system inwhich first, divalent ions (e.g., Ca²⁺ and Mg²⁺) are enriched from brinesolutions using either cross-flow membrane separation or CC. This isfollowed by the subsequent carbonation of enriched solutions. As shownin FIG. 7 a , a feed brine solution from a feed tank is conveyed througha membrane separation unit (as a divalent ion enrichment unit) byoperation of a pump and a valve in a cross-flow mode: a retentate isconveyed back to the feed tank while a permeate passes through amembrane. The feed brine solution is continuously concentrated as thedivalent ions (e.g., Ca²⁺ and Mg²⁺) are retained and accumulate andmonovalent ions (e.g., Na⁺, K⁺, and Cl⁻) in a permeate pass through themembrane. The permeate can be further desalinated by reverse osmosis forwater purification. The concentrated brine solution featuressubstantially reduced volumes (e.g., about 1% to about 10% of the feedsolution) and substantially higher concentrations of the divalent ions(e.g., about 1.2 times or greater than a concentration of the divalentions in the feed solution, such as about 1.4 times or greater, about 1.5times or greater, about 2 times or greater, about 2.5 times or greater,about 5 times or greater, about 10 times or greater, about 20 times orgreater, about 50 times or greater, and up to about 100 times orgreater) is then conveyed to a carbonation reactor (e.g., a stirred-tankreactor) for CO₂ mineralization. FIG. 7 b presents a CC-based enrichmentprocess. As shown, a CC cell (as the divalent ion enrichment unit) caninclude multiple electrodes connected to an electrical source to applyan electrical input across the electrodes. The divalent ions collectedin the electrodes are subsequently released and then flushed directlyinto the carbonation reactor including a small volume of fresh water. Aconcentrated brine solution will have a substantially higherconcentration of divalent ions (e.g., about 1.2 times or greater than aconcentration of the divalent ions in a feed solution, such as about 1.4times or greater, about 1.5 times or greater, about 2 times or greater,about 2.5 times or greater, about 5 times or greater, about 10 times orgreater, about 20 times or greater, about 50 times or greater, and up toabout 100 times or greater) than the initial brine feed.

Following the pre-treatment of the brine, carbonation can be conductedin a stirred-tank reactor by injecting or introducing CO₂-containing gas(e.g., a flue gas) into the concentrated solution. Note that solution pHusually decreases to below about 6 after CO₂ dissolution, which caninhibit carbonate precipitation. Favorable conditions (e.g., a pH ofabout 7 or greater, about 7.5 or greater, or about 8 or greater) can beobtained by adjusting the pH via a pH swing process (e.g., using analkaline reagent in the form of alkaline waste, reusable alkalinebuffer, or resin with ionic exchange groups) so that Ca²⁺ and Mg²⁺ cancontinue to react with CO₂ to form carbonate precipitates. For example,the solution pH can first be elevated to about 11 to about 12 by addingan alkaline reagent (e.g., a leachate derived from alkaline wastes) toallow for a sufficient amount of CO₂ to react with the concentratedbrine solution. As shown in FIG. 8 , nearly about 50% of Ca²⁺ in thelevel of about 10 mM to about 20 mM can be precipitated as carbonatesalts at or near ambient conditions as carbonation proceeds, whilesolution pH is allowed to decrease from about 12 to about 7.5. A seriesof buffering and carbonation processes can be included so as toprecipitate Ca²⁺ and Mg²⁺ at a higher extent (e.g., >about 90%). Whenco-located with a thermal power plant, waste heat from the power plantcan be utilized to increase temperature and/or pressure of a gas feed toenhance the extent and the kinetics of the carbonation reaction. Theaccumulated carbonate salts in the stirred-tank reactor can bethereafter separated from a liquid using a solid-liquid separationmethod, such as filtration.

Some embodiments of this disclosure establish an integrated process fordivalent ion enrichment and subsequent carbonation using industrialand/or natural brine solutions as feed. The enrichment process allowsefficient extraction of divalent ions from brine streams for CO₂mineralization, providing a desirable pathway to reduce the operationalcost and improve the commercial viability of brine mineralizationprocesses. The enrichment process can be implemented in industrial CO₂capture and carbonate production methods that utilize brines as a feedmaterial, such as carbonate production from desalination brines. It canalso be used in brine-based CO₂ capture processes, such as CO₂mineralization of produced water from oil and gas extraction operations.

Compared with other CO₂ sequestration technologies, the brinemineralization process of some embodiments features the followingadvantages:

1) It provides a method for simultaneously treating three waste streams:CO₂, brine waste stream, and/or industrial alkaline waste.

2) The carbon storage capacity is greater as large amounts of brines areavailable, contrary to the finite availability of solid alkalinematerials such as lime, slag and fly ash.

3) The process features a lower energy burden as energy-intensivematerial processing can be omitted, unlike processes utilizing solidminerals like serpentine or olivine that are crushed, milled, and thenheat-treated prior to carbonation.

4) The process reduces the use of alkaline reagents, which can be amajor component of the operational cost in brine mineralizationprocesses. The amount of reagent to carbonate enriched solutions can bemuch smaller than that to carbonate untreated brines. In addition, theuse of an expensive virgin reagent (e.g., NaOH) can be omitted bysubstitution of alkaline wastes.

In summary, the proposed process provides a large-scale CO₂sequestration route that features low energy consumption, operationalstability, reduced chemical use, and scalability.

The following are example embodiments of this disclosure.

First Aspect

In some embodiments according to a first aspect, a method includes: (1)extracting divalent ions from solids by leaching to form a divalention-containing solution; (2) subjecting the divalent ion-containingsolution to concentration to form a concentrated divalent ion-containingsolution; and (3) inducing precipitation of a divalent ion hydroxidesalt from the concentrated divalent ion-containing solution.

In some embodiments of the method, the solids include at least one of(a) slags or (b) fly ashes.

In some embodiments of the method, extracting the divalent ions from thesolids includes exposing the solids to a leaching solution.

In some embodiments of the method, the leaching solution includes aleaching agent.

In some embodiments of the method, the leaching solution includes anacid.

In some embodiments of the method, extracting the divalent ions from thesolids includes pulverizing the solids, and exposing the pulverizedsolids to a leaching solution.

In some embodiments of the method, subjecting the divalention-containing solution to concentration is performed using capacitiveconcentration.

In some embodiments of the method, subjecting the divalention-containing solution to capacitive concentration includes disposingthe divalent ion-containing solution between a pair of electrodes, andapplying an electrical input to the electrodes.

In some embodiments of the method, subjecting the divalention-containing solution to capacitive concentration includes passing thedivalent ion-containing solution through a series of n connectedcapacitive concentration cells, wherein n is 1 or greater.

In some embodiments of the method, inducing the precipitation of thedivalent ion hydroxide salt includes heating the concentrated divalention-containing solution to a temperature at or below about 100° C.

In some embodiments of the method, inducing the precipitation of thedivalent ion hydroxide salt includes heating the concentrated divalention-containing solution to a temperature up to about 90° C.

In some embodiments of the method, the divalent ions include calciumions, and inducing the precipitation of the divalent ion hydroxide saltincludes inducing precipitation of portlandite.

In some embodiments of the method, the divalent ions include magnesiumions, and inducing the precipitation of the divalent ion hydroxide saltincludes inducing precipitation of brucite.

Second Aspect

In some embodiments according to a second aspect, a method forportlandite production includes: (1) extracting calcium from solids byleaching to form a calcium ion-containing solution; (2) subjecting thecalcium ion-containing solution to concentration to form a concentratedcalcium ion-containing solution; and (3) inducing precipitation ofportlandite from the concentrated calcium ion-containing solution.

Third Aspect

In some embodiments according to a third aspect, a method for bruciteproduction includes: (1) extracting magnesium from solids by leaching toform a magnesium ion-containing solution; (2) subjecting the magnesiumion-containing solution to concentration to form a concentratedmagnesium ion-containing solution; and (3) inducing precipitation ofbrucite from the concentrated magnesium ion-containing solution.

Fourth Aspect

In some embodiments according to a fourth aspect, a method includes: (1)producing a concentrated divalent ion-containing solution from a feedsolution; and (2) introducing carbon dioxide to the concentrateddivalent ion-containing solution to induce precipitation of a carbonatesalt.

In some embodiments of the method, production of the concentrateddivalent ion-containing solution is performed by nanofiltration orreverse osmosis. In some embodiments, production of the concentrateddivalent ion-containing solution is performed using a filtrationmembrane with a pore size from about 0.5 nm to about 10 nm. In someembodiments, production of the concentrated divalent ion-containingsolution includes production of a retentate having a greaterconcentration of divalent ions than the feed solution.

In some embodiments of the method, production of the concentrateddivalent ion-containing solution is performed by capacitiveconcentration.

In some embodiments of the method, a concentration of divalent ions inthe concentrated divalent ion-containing solution is about 1.2 times orgreater than a concentration of the divalent ions in the feed solution,such as about 1.4 times or greater, about 1.5 times or greater, about 2times or greater, about 2.5 times or greater, about 5 times or greater,about 10 times or greater, about 20 times or greater, about 50 times orgreater, and up to about 100 times or greater.

In some embodiments of the method, a concentration of monovalent ions inthe feed solution is greater than a concentration of divalent ions inthe feed solution. In some embodiments, a concentration of the divalentions in the concentrated divalent ion-containing solution is greaterthan a concentration of the monovalent ions in the concentrated divalention-containing solution, such as about 1.2 times or greater, about 1.4times or greater, about 1.5 times or greater, about 2 times or greater,about 2.5 times or greater, about 5 times or greater, about 10 times orgreater, about 50 times or greater, about 100 times or greater, about500 times or greater, and up to about 1000 times or greater.

In some embodiments, the method further includes adjusting the pH of theconcentrated divalent ion-containing solution, prior to introduction ofthe carbon dioxide. In some embodiments, adjusting the pH of theconcentrated divalent ion-containing solution includes increasing the pHto about 7 or greater, about 7.5 or greater, about 8 or greater, about 9or greater, about 10 or greater, or about 11 or greater. In someembodiments, adjusting the pH of the concentrated divalention-containing solution includes introducing an alkaline reagent to theconcentrated divalent ion-containing solution. In some embodiments, thealkaline reagent includes an alkaline waste.

In some embodiments of the method, introduction of the carbon dioxideincludes inducing precipitation of at least one of the following:calcium carbonate, magnesium carbonate, or other carbonates (e.g.,barium carbonates) or other related solids.

Fifth Aspect

In some embodiments according to a fifth aspect, a system includes: (1)a divalent ion enrichment unit to produce a concentrated divalention-containing solution from a feed solution; and (2) a carbonationreactor connected to the divalent ion enrichment unit to receive theconcentrated divalent ion-containing solution and introduce carbondioxide to the concentrated divalent ion-containing solution to induceprecipitation of a carbonate salt.

In some embodiments of the system, the divalent ion enrichment unitincludes a membrane separation unit. In some embodiments, the membraneseparation unit includes a nanofiltration membrane. In some embodiments,the membrane separation unit includes a reverse osmosis membrane.

In some embodiments of the system, the divalent ion enrichment unitincludes a capacitive concentration cell. In some embodiments, thecapacitive concentration cell includes multiple electrodes and anelectrical source connected to the electrodes.

As used herein, the singular terms “a,” “an,” and “the” may includeplural referents unless the context clearly dictates otherwise. Thus,for example, reference to an object may include multiple objects unlessthe context clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or moreobjects. Thus, for example, a set of objects can include a single objector multiple objects. Objects of a set also can be referred to as membersof the set. Objects of a set can be the same or different. In someinstances, objects of a set can share one or more commoncharacteristics.

As used herein, the terms “connect,” “connected,” and “connection” referto an operational coupling or linking. Connected objects can be directlycoupled to one another or can be indirectly coupled to one another, suchas via one or more other objects.

As used herein, the terms “substantially” and “about” are used todescribe and account for small variations. When used in conjunction withan event or circumstance, the terms can refer to instances in which theevent or circumstance occurs precisely as well as instances in which theevent or circumstance occurs to a close approximation. For example, whenused in conjunction with a numerical value, the terms can refer to arange of variation of less than or equal to ±10% of that numericalvalue, such as less than or equal to ±5%, less than or equal to ±4%,less than or equal to ±3%, less than or equal to ±2%, less than or equalto ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, orless than or equal to ±0.05%.

Additionally, concentrations, amounts, ratios, and other numericalvalues are sometimes presented herein in a range format. It is to beunderstood that such range format is used for convenience and brevityand should be understood flexibly to include numerical values explicitlyspecified as limits of a range, but also to include all individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly specified. For example, arange of about 1 to about 200 should be understood to include theexplicitly recited limits of about 1 and about 200, but also to includeindividual values such as about 2, about 3, and about 4, and sub-rangessuch as about 10 to about 50, about 20 to about 100, and so forth.

While the disclosure has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the disclosure asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, operation or operations, to the objective, spirit and scope ofthe disclosure. All such modifications are intended to be within thescope of the claims appended hereto. In particular, while certainmethods may have been described with reference to particular operationsperformed in a particular order, it will be understood that theseoperations may be combined, sub-divided, or re-ordered to form anequivalent method without departing from the teachings of thedisclosure. Accordingly, unless specifically indicated herein, the orderand grouping of the operations are not a limitation of the disclosure.

What is claimed is:
 1. A method comprising: extracting divalent ionsfrom solids by leaching to form a divalent ion-containing solution;subjecting the divalent ion-containing solution to concentration to forma concentrated divalent ion-containing solution; and inducingprecipitation of a divalent ion hydroxide salt from the concentrateddivalent ion-containing solution, wherein subjecting the divalention-containing solution to concentration is performed using capacitiveconcentration.
 2. The method of claim 1, wherein the solids include atleast one of (a) slags or (b) fly ashes.
 3. The method of claim 1,wherein extracting the divalent ions from the solids includes exposingthe solids to a leaching solution.
 4. The method of claim 3, wherein theleaching solution includes a leaching agent.
 5. The method of claim 3,wherein the leaching solution includes an acid.
 6. The method of claim1, wherein extracting the divalent ions from the solids includespulverizing the solids, and exposing the pulverized solids to a leachingsolution.
 7. The method of claim 1, wherein subjecting the divalention-containing solution to capacitive concentration includes disposingthe divalent ion-containing solution between a pair of electrodes, andapplying an electrical input to the electrodes.
 8. The method of claim1, wherein subjecting the divalent ion-containing solution to capacitiveconcentration includes passing the divalent ion-containing solutionthrough a series of n connected capacitive concentration cells, whereinn is 1 or greater.
 9. The method of claim 1, wherein inducing theprecipitation of the divalent ion hydroxide salt includes heating theconcentrated divalent ion-containing solution to a temperature at orbelow 100° C.
 10. The method of claim 1, wherein the divalent ionsinclude calcium ions, and inducing the precipitation of the divalent ionhydroxide salt includes inducing precipitation of portlandite.
 11. Themethod of claim 1, wherein the divalent ions include magnesium ions, andinducing the precipitation of the divalent ion hydroxide salt includesinducing precipitation of brucite.